Mangold, as good as he is, isn't Spielberg.
I don't think we have to try hard to imagine what Indy would act like as an old man. We had it already. It was Henry Sr. who was still enthusiastic about archeology and finding the next adventure. It seemed to me him and Indy were more alike than different.
I would imagine Indy at this point in his life is somewhat of a celebrity. His exploits have surely become the stuff of legend. His classroom would still be filled with a lot of very interested students but, instead of girls fawning over him, now it's aspiring historians and archeologists who have an almost fanboy-like obsession with him probably to the point where they show up wearing fedoras and leather jackets. Indy finds their fanaticism very silly but appreciates their enthusiasm, so much so, that he often takes them out to the university quad for bullwhip lessons. He's still together with Marion, Mutt is still alive, and everything is peaches. However, he still feels a longing in his heart to be out in the field (ala Kirk in TWOK). The movie is set at the end of the space race. You have great symbolism to work with there. The world is excited for what the future holds and most people are not looking to the past anymore. Indy, feeling as though he's lost his relevancy, is desperate to find that last adventure and it comes calling in the form of a grown-up Short Round. Why Short Round? Because it's a character we have a history with and care about as opposed to Helena. Is it a cheap member-berry tactic to bring him back? Absolutely, but so is bringing back an 80 year old Indiana Jones so might as well go full bore. Sallah shows up. Mutt tags along. One last hurrah.
There you go. You give him an internal dilemma while still maintaining a positive state of existence and reverence.
Edit: Happy 4th